The Vapor-phase Axial Deposition (VAD) method of fabricating multimode optical fiber is described in U.S. Patent 3,966,446 which issued on June 29, 1976 and is assigned to Bell Telephone Laboratories. The VAD process begins with the deposition of germanium phosphosilicate soot particles, which are formed by the hydrolysis of SiCl.sub.4, GeCl.sub.4, and POCl.sub.3 in an oxyhydrogen flame, on the end of a rotating bait rod. The bait rod is slowly moved away from the torch resulting in a substantially cylindrical, porous soot-form. The porous soot-form is subsequently dehydrated and consolidated at an elevated temperature into a clear glass boule. The consolidated glass boule is then stretched into an elongated rod that is to be used as the core of a preform from which multimode lightguide fiber is drawn. The cladding of the preform is formed by the so-called "rod-in-tube" method, which entails the collapse of a silica tube over the core rod which is inserted therein.
In the VAD method for producing single-mode lightguide fiber preforms, it is necessary to produce a small core upon which several cladding layers are deposited. To achieve an optimum clad-to-core ratio of 15:1 in one deposition step while maintaining manageable soot-form size, the core diameter should not be any larger than 18 mm (soot). Under growth conditions, the porous core material should be of sufficient density to support the heavy cladding layers deposited upon it. Also, the core soot should be uniform radial density in order to avoid bubble formation at the clad-to-clad interface upon consolidation.
In fabricating single-mode optical fibers by the VAD method, as described in U.S. Pat. No. 4,345,928 to Kawachi et al, which is incorporated by reference herein, the cladding material cannot be fabricated solely by the rod-intube method described above. Because of the small diameter of the single mode fiber core (e.g., 5 to 10 .mu.m), part of the lightwave traveling therealong finds passage in a portion of the clad near the core. The rod-in-tube method of fabricating all the cladding material results in a fiber that has high attenuation due to the presence of OH in the cladding. Thus, for low-loss, single-mode fibers made by the VAD Method, part of the cladding material must be fabricated, like the core, with a soot deposition step to be followed by dehydration before consolidation into a clear glass lightguide preform.
Typically, the amount of cladding material to be fabricated in this manner must yield a soot-form that has a deposited clad-to-core diameter ratio higher than 6 to 1. Thus, for single-mode fiber fabrication, the VAD method entails the use of several oxy-hydrogen torches for simultaneous soot formation; one torch for the axial deposition of a core and one or more torches to radially deposit the clad. After dehydration and consolidation, the resulting clear glass boule is also stretched and extra cladding material is added by the above-described rod-in-tube method to attain the proper clad-to-core diameter ratio to give the desired cutoff wavelength. Typically, for a 125 .mu.m outside diameter fiber with a cutoff wavelength of 1.15 .mu.m and a step index of .DELTA.n=0.004, the core has a diameter of approximately 8 .mu.m.
In multimode VAD fabrication, a circular cross-section torch comprised of five concentric, spaced quartz tubes is used to make the soot-forms for fabricating the core of the preform. Two inner tubes deliver the chemicals, SiCl.sub.4, GeCl.sub.4, and POCl.sub.3, while the other three tubes supply H.sub.2, Ar and O.sub.2 for the oxyhydrogen flame. Typically, a soot-form that is made with this torch configuration has a core diameter on order of 50 mm. If a similar torch configuration is employed for single-mode fiber fabrication, the overall soot-form which encompasses both the core and clad would be overwhelmingly large (e.g., 300 mm). Such large soot-forms have built-in stresses making handling and sintering extremely difficult. Thus, for single-mode fiber fabrication, a technique must be developed to fabricate the core of the single-mode soot-form having a diameter any larger than 18 mm.
U.S. Pat. No. 4,465,708 to Fanucci et al, which is also incorporated by reference herein, is directed to a torch used in the VAD process to form small cores for single mode lightguide preforms. The torch is comprised of a plurality of concentrically mounted, spaced glass tubes through which reactants and combustible materials are passed. A tapered shroud is mounted about the end of the torch and shield gas is directed along the inner surface thereof to confine and direct the gases and reactants to the surface of the growing soot-form.
Although this technique is effective in producing small diameter cores (e.g., 16.5 mm), the refractive index and density profiles can be adversely affected due to the fact that the shroud may produce a non-uniform temperature distribution on the core surface. This process results in a core that has a hot (approximately 900.degree. C.) tip and progressively cooler regions as one moves upward along the surface. Such a thermal pattern can cause a dense, germanium-depleted area to form at the center of the core. As one progresses radially toward the edge, the germanium increases significantly and the density decreases significantly to the point at which the outer surface of the core is a very loosely bound, fluffy layer of germania rich soot particles. This fluffy layer can cause problems, not only during boule growth, but during consolidation as well. During growth, this loosely bound layer of soot may not be capable of supporting the weight from cladding deposition before cracking. Furthermore, during consolidation, the loose layer of germania rich soot can cause bubbles to form at the clad-to-core interface, thus rendering some of the resulting preforms useless for fiber production.